The present invention relates to a thermopneumatic actuator especially suited for controlling the temperature in the passenger compartment of an automotive vehicle.
Passenger automobiles and trucks are normally provided with heating and cooling systems for maintaining the desired temperature in the vehicle passenger compartment. The temperature is controlled by selectively energizing the heating and cooling systems and furthermore by positioning a temperature door which controls the mixture of hot and cold air.
The temperature door is generally positioned by means of a vacuum actuator which is fed with modulated vacuum from the vehicle intake manifold. The actuator typically comprises a diaphragm which is positioned by opposing forces of the modulated vacuum and a diaphragm spring. The vacuum actuator may be provided with a power diaphragm and a pilot diaphragm to increase the accuracy and decrease the effects of variations in the mechanical resistance encountered in moving the temperature door.
The vacuum to the actuator is modulated by means of a thermally controlled valve. A bimetal spring exposed to air from the passenger compartment positions a valve element to bleed air into the vacuum actuator and thereby reduce the vacuum as a function of temperature. Such a thermally controlled valve is generally referred to in the art as a thermostatic vacuum regulator.
A system of this type is an open loop control system since there is no mechanical feedback between the diaphragm and the valve. Thus, the system is inherently inaccurate since such factors such as variations in the vacuum applied to the regulator, ageing of the diaphragm, pressure drops between the regulator and the actuator and the like will cause the temperature to be controlled in an erratic manner. In addition, the system is disadvantageous from an installation standpoint since the regulator and actuator are separate units. They must be mounted in separate locations and connected by a conduit, constituting unnecessary consumption of mounting space, installation time and expense.
Yet another drawback of such a prior art system is a major cause of poor temperature regulation. The operation of heaters and coolers in automotive air conditioning systems tends to vary in response to engine speed, load, cyclic operation of the cooler compressor and similar factors. Thus, with temperature door positioned as a function only of temperature in the compartment, the temperature of the air being fed into the compartment may vary substantially. Due to the long response time inherent in such a system, the result is erratic operation.